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A large bucket index can lead to performance problems. In order
to address this problem we introduced bucket index sharding.
Until Luminous, changing the number of bucket shards (resharding)
needed to be done offline, from Luminous we support
online bucket resharding.

Each bucket index shard can handle its entries efficiently up until
reaching a certain threshold of entries. If this threshold is exceeded the system
could encounter performance issues.
The dynamic resharding feature detects this situation and increases
automatically the number of shards used by the bucket index,
resulting in the reduction of the number of entries in each bucket index shard.
This process is transparent to the user.

The detection process runs:
1. When new objects are added to the bucket
2. In a background process that periodically scans all the buckets
This is needed in order to deal with existng buckets in the system that are not being updated.
A bucket that requires resharding is added to the reshard_log queue and will be
scheduled to be resharded later.
The reshard threads run in the background and execute the scheduled resharding, one at a time.